1. Field of the Invention
The present invention relates to methods and apparatus for the recording of digital broadcast material and in particular to the recording of multimedia applications accompanying television broadcasts.
2. Discussion of Related Art
A broadcaster can broadcast multimedia platform-specific applications possibly together with digital television programs. A suitably equipped multimedia platform-specific set-top box can receive those applications and run them locally. Example applications are electronic program guides, play-along games, Tele-banking, Tele-shopping, electronic newspapers and similar information services. Television programs can be recorded and, if such a television program has an application associated with it, then that application should also be recorded. Typically multimedia platform-specific applications are broadcast in an object carousel, where all the application code and data is broadcast in cycles. This resembles teletext data, which is also broadcast in a carousel.
A suitable transmission system for such application delivery is known from ISO/IEC International Standard 13818-6, “MPEG-2 Digital Storage Media Command and Control” Jul. 12, 1996 (identified herein as DSM-CC). In modern digital broadcast systems a transmitter typically transmits a large number of services (or channels) to a plurality of receivers, examples of which are to be found in television sets or set-top boxes. Such a service can contain an audio/video stream, an interactive application (for example in the MHEG-5 format), other kinds of data or a combination of these elements. An MPEG-2 transport stream is a multiplex of a number of services, and a transmitter will typically transmit several transport streams to the set-top boxes. In turn, a suitably configured set-top box can tune to a specific transport stream and is then able to retrieve information from that transport stream.
As mentioned above, interactive multimedia applications are typically broadcast in a carousel-like fashion with successive data sections being repeated periodically and sequentially in the transport stream. For instance, both DVB and DAVIC have specified DSM-CC object carousels, as mentioned above, for broadcasting interactive applications.
As is described in the commonly-assigned International patent application WO 99/65230, the objects of a DSM-CC object carousel are broadcast in modules and provide a “virtual” file system comprised of file and directory objects in the manner of a personal computer (PC) file system. Such a module is a container of objects and comprises a number of DownloadDataBlock messages (which are specified in the MPEG-2 standard as private sections). When a set-top box wants to pre-fetch a DSM-CC object, it must (amongst other things) know in which module the object resides. After it has retrieved the right module, the set-top box must then parse the module to get to the object itself. Due to the hierarchical nature of the DSM-CC object carousel an object might be included in a subdirectory. If this is the case, the set-top box must also retrieve the module(s) with the intermediate directories, and parse them before it gets to the object in which it is interested.
Typically, the service provider will broadcast the object carousel in a compressed form. This compression is normally done at the module level. Thus, retrieving an object requires also the decompression of all the modules that are needed for the retrieval of the objects the set-top box is interested in. As will be recognised, the hierarchical nature of the DSM-CC object carousel for the purpose of pre-fetching objects requires a lot of processing in the set-top box. Consequently, when considering the issue of recording as an adjunct to capture of digital video broadcasts, it will be recognised that there is a lack of an efficient way to record (and play-back) object carousels.
In such set-ups, the module is the unit of transmission, and it is thus not possible to send a part of a module; either the entire module is sent or nothing is sent. Furthermore, the module is the unit of packaging, with the objects in a module being typically compressed together.
File and directory objects can change over time. In the DSM-CC object carousel, versioning is not done at the (file and directory) object level but at the module level. Only modules have version numbers. Even if only one object in a module changes, the complete module gets a new version number.
Amongst the characteristics of modules and objects is that the grouping of objects in modules does not need to be constant over time. Objects can be moved between modules, and objects can be added and removed.
Since modules are broadcast in MPEG-2 transport streams, and each module is broadcast in the private data sections of an elementary stream, then typically a large number of modules will share the same elementary stream and a complete object carousel will generally be carried on only a limited number of elementary streams (typically fewer than 5).
As will be recognised by the skilled practitioner, the object carousel consists of three layers, wherein the top layer consists of the file and directory objects, the layer below that consists of modules, and the layer below that consists of private data sections in an elementary stream.